Scientists Discover World’s Largest Seaweed Bloom

Brian Lapointe, Ph.D., co-author and a research professor at FAU’s Harbor Branch, emerges from a seaweed bloom. Lapointe has studied Sargassum for more than four decades, and is among a team of scientists who have discovered the world’s largest macroalgae bloom.

In just the right amounts, Sargassum – a floating tropical brown seaweed – is essential for marine life. Too much, however, can be disastrous, causing serious environmental, ecological and economic problems. Frequent beaching events have been seen in Florida and the Caribbean since 2011, and these unusually thick mats of seaweed are now infesting and stinking up these once pristine beaches.

Until 2011, the frequency and extent of these large influxes of Sargassum were not observed in the Caribbean region. The source of these massive blooms have remained a mystery to scientists due to a lack of large-scale data.

The “Great Atlantic Sargassum Belt” extends from West Africa to the Gulf of Mexico. Last year, the massive 8,850-kilometer bloom contained more than 20 million tons of Sargassum biomass. This recurrent bloom and beaching events may just become the “new normal,” according to a study published in Science.

In addition to satellite data, the team used environmental and field data to suggest that the belt forms seasonally in response to two key nutrient inputs: one human-derived, and one natural. In the spring and summer, Amazon River discharge adds nutrients to the ocean, and such discharged nutrients may have increased in recent years due to increased deforestation and fertilizer use. In the winter, upwelling off the West African coast delivers nutrients from deep waters to the ocean surface where the Sargassum grows.

To unravel the mystery, the team analyzed fertilizer consumption patterns in Brazil, Amazon deforestation rates, Amazon River discharge, two years of nitrogen and phosphorus measurements taken from the western parts of the central Atlantic Ocean near the Amazon discharge, among other ocean properties. While the data are preliminary, the pattern seems clear: the explosion in Sargassum correlates to increases in deforestation and fertilizer use, both of which have increased since 2010.

“Severe floods have recurred in the Amazon basin since 2009, which would result in extensive freshwater runoff and nutrient enrichment in the western Atlantic Ocean,” said Lapointe. “It is reasonable to suggest that the 2011 massive bloom is therefore the result of nutrient accumulations since 2009, resulting from stronger upwelling in the eastern Atlantic and excessive Amazon River discharges in the western Atlantic.”

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The conditions that appear to be associated with massive Sargassum blooms at magnitudes comparable to those in 2015 and 2018, are large seed populations during winter as a result of the previous year’s bloom, high nutrient supply from the West Africa upwelling in winter months, and higher nutrient supply from the Amazon River input but normal or lower sea surface temperatures during the current years. If these conditions are met, then a massive bloom is likely to occur in the central Atlantic, followed by severe beaching events in the Caribbean Sea in later months.

“The significant biomass accumulations along the pathway of the Great Atlantic Sargassum Belt underscore the need for multidisciplinary research to better understand their ecological and biogeochemical impacts as well as their impacts on coastal environments, tourism, economies, and human health, especially when the role of Sargassum changes from an essential habitat to a significant and perpetual nuisance,” said Lapointe.

This work is funded by the U.S. NASA Ocean Biology and Biogeochemistry Program (NNX14AL98G, NNX16AR74G) and Ecological Forecast Program (NNX17AE57G), NOAA RESTORE Science Program (NA17NOS4510099), the JPSS/NOAA Cal/Val project (NA15OAR4320064), the National Science Foundation (NSF-OCE-0934025 and NSF-OCE-1737078), and by a William and Elsie Knight Endowed Fellowship.

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